The invention relates generally to the field of sensors, and particularly to sensors for measuring relative displacement between rotating and moving components in a rotating machine. Specific embodiments of the present technique provide a system and method for sensing radial clearance and relative axial displacement of the rotating components, such as blades and seal tips with respect to the stationary shroud in a turbine engine.
Efficiency and performance of gas and steam turbines are affected by clearances between tips of the rotating blades and the stationary shrouds. In a turbine stage, the portion of the working fluid that passes through the clearance between the tips of the turbine blades and the static shroud does no work on the rotor blades, and hence leads to a reduced work efficiency of the turbine stage. Hence, in the design of gas and steam turbines, it is desirable to have a close tolerance between the tips of the rotor blades and the surrounding static shroud. In certain turbine stages, tips of the rotating blades have continuous knife-edges (also known as seal teeth or packing teeth) that mesh corresponding grooves on the stationary shroud to provide sealing of the gas path between the rotating blades and the shroud.
However, clearance dimensions between the rotating blades and the shroud vary during various operating conditions of the turbine engine. A significant reason for this is the dissimilar thermal growth within the engine between the blade tips and the shroud surrounding them, for example, during start-up of the turbine engine. In such a case, the high temperature of the working fluid causes the blades to be at a higher temperature than the surrounding shroud. As a result, the blades expand radially with respect to the shroud and the clearance between the shroud and the blades decreases, which may cause the interfacing surfaces of the blades and the shroud to rub, resulting in an increased engine cold start-up time. Further, during such thermal transients, the blades expand axially with respect to the shroud, which may lead to interference of the blade tips with grooves or rails on the stationary shroud.
In order to facilitate lower engine start-up times and to avoid rubbing or interference between the static and rotating components, it is desirable to obtain an accurate on-line or real-time measurement of the radial and axial displacements at various turbine stages to insure that radial and axial clearances are maintained. However, currently, clearances between the blades or seal teeth and the shroud are generally determined based on thermal models, which may often be inaccurate. The use of models to determine clearances may result in a relatively long warm-up time before a turbine may be started because model prediction results cannot be relied on to be accurate. Thus, turbine users tend to add extra time to the model prediction results in an effort to help ensure that adequate clearance exists prior to turbine start-up. Further, no technique currently exists to measure axial clearance at the turbine stages. Generally, the axial displacement of the blades with respect to the shroud is measured at the exit of the turbine by correlating the displacement of the entire shell at one end and the displacement of the rotor at that same end. The above technique may prove ineffective because growth of individual stages need not be essentially linear due to bulk of material distributed around certain stages form a bigger thermal sink than the rest of the engine.
Accordingly, there is a need for a system and method for on-line and unified measurement of radial and axial displacements within the turbine engine at a given stage.